In a phylum dominated by marine species as diverse as the jellyfishes and corals, the hydra (class Hydrazoa) are among the few commonly observed coelenterates in freshwater ecosystems. Skipping the medusa stage typical of the jellyfish, the stalked and tentacled invertebrate polyps can reproduce sexually or asexually. They follow the basic coelenterate radially symmetric body plan of two cell layers without a true body gut, having instead a gastrovascular cavity, with the mouth as a single opening.

Hydra bear the name of the Greek mythological creature of Lerna, described as having the body of a hound and 100 serpentine heads, each bearing poisonous breath. With so hideous a visage that many died of fear just viewing the mythological beast, the freshwater hydra has much legend to live up to. Hydras are easily identified by their numerous tentacles, each bearing poisonous nematocysts that can paralyze or kill prey. The tentacles are retractable and bear up to four different types of stinging cell. For such a primitive and simple creature, the nematocysts are among the most advanced and intricate organs developed in the aquatic realm. Some nematocysts function as harpoons, with large barbs that anchor the tentacle to the prey so that it can be reeled in, delivering the prey to the hydra's other tentacles and mouth. Another small nematocyst type features a thicker and shorter corkscrew thread that wraps around and grips the prey. A sticky, bean-shaped object is found on the end of the third type of stinging cell, and functions as an aid in locomotion. Probably used in the defense of the hydra, the fourth type of nematocyst has spines running the length of the thread.

Without circulatory, respiratory, or excretory systems, the hydra's cells depend on simple diffusion to deliver respiratory gases and ingested matter for nutrition, and these freshwater coelenterates have no functional need for blood. Asexual reproduction by the hydra occurs through budding, or regeneration. A hydra can be cut entirely in half longitudinally, and it is possible for two hydras to grow, each a genetically identical clone. In the process referred to as budding, a new individual grows from the parent's body. Once it develops tentacles and its own foot, the offspring breaks off to grow as a new hydra clone. Prior to the winter, hydras develop egg and sperm cells instead of budding, and following fertilization, a developing zygote settles into the soft aquatic sediments. There, it overwinters while the parents die in the cold waters of temperate and polar regions. In many cases, the individual hydra is hermaphroditic, producing both male and female cell types.

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